Generators and switch mode power supplies.

Could be, I suppose, though whether the generator can keep its RPM constant (or rather, within acceptable bounds), would depend on how fast it can change its throttle setting.

Sylvia.

"JosephKK"

** Not in the context or electronic loads on the AC supply - pal.

Here they are then one and the same.

** The WHOLE PURPOSE of an active PFC circuit is eliminate harmonic currents from the AC supply - pure luck that making the current draw a sine wave corrects the power factor too.

..... Phil

nic

w a

Gee folks it's all right here

I quote: In circuits having only sinusoidal currents and voltages, the power factor effect arises only from the difference in phase between the current and voltage. This is narrowly known as "displacement power factor". The concept can be generalized to a total, distortion, or true power factor where the apparent power includes all harmonic components. This is of importance in practical power systems which contain non-linear loads such as rectifiers, some forms of electric lighting, electric arc furnaces, welding equipment, switched-mode power supplies and other devices.

so reducing harmonic currents DOES actually improve power factor.

Mark

"JosephKK"

Gee folks it's all right here

** You should have quoted this section on ACTIVE PFC.

I quote: In circuits having only sinusoidal currents and voltages, the power factor effect arises only from the difference in phase between the current and voltage. This is narrowly known as "displacement power factor". The concept can be generalized to a total, distortion, or true power factor where the apparent power includes all harmonic components.

** Pedantic drivel.

The DEFINITION of "power factor " is simply

PF = watts / VA.

Both the V and A in " VA" are defined as being the * rms values * of the quantities.

So, when the current waveform is peaky instead of sine wave, it's rms value increases significantly for the same power consumption.

So VA gets bigger than watts and hence the PF is poor.

..... Phil

f the

ue

you just agreed with me..

I said removing harmonics DOES actually improve power factor.

Mark

"TheM" skrev i meddelelsen news:Xafil.777$ snipped-for-privacy@news.siol.net...

Anything with a rectifier/capacitor input will not like square-wave much - the peak currents in the rectifiers will be very large! Probably squares will also mess with the tiny silicon brain of any power factor correction logic . ;-)

A very good idea IMO.

If the idea was any good, you wouldn't have to impose it by force of arms.

Thanks, Rich

All the circuits I've seen have had a resistor in series to limit the inrush current at turn on, and this resistor would also limit rectifier currents when the gear is operated on a square wave.

Probably

I certainly wondered about that. The specs for my UPS (A Sola 325) say "Load Compatability: Can support 100% power factor corrected, switch-mode power supply load."

But when running on its batteries, it produces what looks to be a bulk standard "modified sine wave".

Sylvia.

It's a classic example of an external cost. Those using equipment that imposes harmonic losses on the infrastructure don't pay for the costs they cause. Instead, those costs are shared by all users of the infrastructure.

Markets almost never internalise external costs unless they are forced to, regardless of the overall benefits of doing so. It takes legislative action.

So the notion that the idea wouldn't have to be imposed if it were any good is false.

Sylvia.

PFC prevents fires, too. You approve of fires?

John

"Sillier than Anyone Else"

** That resistor will likely be sized to withstand the heat generated by a few switch on cycles per minute.

BUT with a square shaped supply voltage, there will be 100 ( or 120 in the USA) short current surges per SECOND !!

The rms value of such a current pulse stream is very high and so smoke the resistor and or other components like diodes, filter chokes and filter electros.

As the use of "modified square wave" in UPS designs is standard AND the use of UPSs with PCs very common, all PC supplies and accessories have to be able to tolerate this scenario.

But there is no obligation on the maker's of other classes of device employing SMPS to allow for it at all.

..... Phil

Almost right. Profit seeking organizations (especially corporations) and even some not-for-profit organizations have an obvious vested interest in externalizing all costs possible. When these organizations participate in a market they seek to maximize the externalization of cost by pushing those costs onto the purchasers of their products and services. This is why mining, refining, manufacturing, and many other organizations were (and are in China) so cavalier about pollution and using/selling substandard materials. The organization is purely chasing the (the most result/effect from the) holy yuan/franc/mark/dollar/peso as demanded by the stakeholders/stockholders. When the problems created by externalizing the costs of doing things most cheaply instead of cleaner ways, the market reacts with many tools, including boycotts, mass plaintiff lawsuits and driving government regulation and legislation. Unfortunately, governments being what they are rarely hit the mark and usually under/over regulate.

In the context of harmonic loads, the issue is essentially technical. Beyond second order effects such as needing to mine more metal to make the transmission cables, there wouldn't be much for consumers to care about, except that they have to pay more for electricity. I can't see market action such as boycotts and the like coming to pass. Few consumers are even going to hear about the issue, let alone understand it.

Sylvia.

The resistor is sized to handle the energy it has to accept during the turn on inrush. That isn't a power issue. However, given that such resistors are typically physically quite large, they also have an ability to dissipate a reasonable amount of power.

On the face of it, a true square wave input wouldn't trouble the resistor in a basic SMPS at all, since the resistor would be dissipating less power than under normal operation. A rectified square wave is essentially pure DC.

With a modified sine wave, it comes down to how much sag occurs on the capacitor during the time the input is zero. The greater the sag, the more likely it is that the resistor will fail.

I can't see why the current should trouble rectifiers. They have to handle the inrush current, as limited by the resistor, and their power dissipation is mainly a function of the total charge going through them, not current waveform.

Once you bring in filter electronics, all bets are off.

Sylvia.

Well, you go right ahead then - The Whip Teaches (some people)

"Sillier than Anyone Else"

** Of course it IS a POWER RATING issue

- you crazy bitch.

** Correct.

The worst case scenario is a small SMPS running on a high capacity UPS.

SMPS that use an NTC thermistor instead of a resistor are in even more trouble.

** Just a singe, large current surge with plenty of time to cool afterwards.

Power rectifier diodes have large, single surge ratings.

But you have never looked up the maker's specs.

** BOLLOCKS.

Diode heat dissipation at mains frequencies is entirely a function of current.

** Does not stop you posting more pedantic tripe though - does it???

..... Phil

Almost right.

To externalise costs, you have to push them onto the general public. Pushing costs only onto your customers puts you at a competitive disadvantage.

If legislation pushes a cost onto the manufacturer, it becomes a manufacturing cost, no different to the cost of raw materials or labour. Whether it gets pushed onto the customer or eats into the manufacturer's profit margin depends upon the degree of competition.

The resistors used in such circuits have ratings in Joules, reflecting the fact that the problem is one of energy, not power.

Yup. Have. They tend to talk about average current.

Some amount of charge is removed from the capacitor during a half-cycle. That charge has to be replaced over the period of a half-cycle, and has to come through the rectifier. The energy dissipated in the rectifier in that time is the charge times the rectifier's forward voltage. The latter of course is not totally fixed, but rises somewhat with current, so the power dissipation also rises somewhat if the charge is transfered in a shorter time. All the same, unless the rectifier is normally being run close to its thermal limit (which would hardly be good design), I would expect it to survive.

Sylvia.

Where all the competing products were previously externalising the cost, and all now have to internalise it, the cost must inevitably be passed on to the consumer, because there's no change in competition.

Sylvia.